Hegemann found herself the focus of a campaign spearheaded by Günter Grass to protect writers’ intellectual copyright. Photograph: Helene Hegemann

No one seems to recognise Helene Hegemann as she bounces into the busy cafe, small, slight, dressed in a grey velour tracksuit, brushing her thick hair from her face, and plonking her battered iPhone on the table. Yet this is a woman who, two years ago, took Germany's literary establishment by storm before being just as swiftly knocked off her pedestal by the same intellectual elite when it was discovered she had lifted parts of her debut novel from unattributed sources.

But this is Berlin, where it is considered cool not to notice, or admit you've noticed, the famous.

Or perhaps it's also to do with the fact that Hegemann, whose novel Axolotl Roadkill, about a troubled, precocious and brassy 16-year-old girl who has more or less been left to fend for herself in the raw seediness and creative confusion of Berlin (which was written when she was 16 and published when she was 17), has been trying to keep a low profile and so seems happy to blend in.

"It was a totally strenuous time. I was completely exposed," the now 20-year-old tells me over a ginger tea. "I was reading articles where I thought 'what a horrible person', and then realising with horror, 'it's me they're talking about'."

When the book came out, Hegemann appeared on all the top chatshows and in the arts pages of every German newspaper, being thrust, she says, into "a completely new world, part of a hype that was not about the book itself or its contents, but about the fact that I was just 17 and had written about grisly things". Then the plagiarism claims started and the exposure was more intense still.

At the height of the furore, Hegemann was the focus of a campaign spearheaded by German literary giant Günter Grass to protect writers' intellectual copyright. Some critics went so far as to suggest the novel had been written by her father. She responded with a barbed essay in Die Zeit in which she joked yes, her father had written it, but only after she had slept with him.

We're sitting in the glass-walled cafe of Berlin's KW Institute for Contemporary Art, on a gallery-strewn street that features heavily in her novel, as does the now highly gentrified district of Prenzlauer Berg, favoured by Brits who have moved to the city in droves in recent years.

While she does not like it to be thought of as a Berlin novel, she is aware this will attract potential British readers, in an age when young Britons think nothing of flying to Berlin for a single night to hit the clubs (including the notorious Berghain, where much of the book is set).

"Berlin has certainly shaped me," she says. But while she found the city liberating when she arrived from the industrial city of Bochum at the age of 13, it was almost too free and wild. "I was out of my depth," she says. Her mother had recently died, and her father, Carl Hegemann, a dramaturg at Berlin's experimental theatre, the Volksbühne, was supposed to take her under his wing, but by all accounts didn't quite know how to deal with her. "Really, for years all I knew about my father was that he worked at a theatre far, far away but I hardly ever saw him and had never visited. Let's just say when I arrived in Berlin it was a shock and I did not experience a conventional family life."

She found the adjustment hugely difficult. She skived off school, hung out with people of every age, background and persuasion, and eventually, at 15, got an unwritten ("silent, angry") agreement with the authorities and her father that she did not have to return to school. "I wasn't rejecting the idea of learning or education as such, I simply didn't fit into the system. I told them all: 'If you force me to carry on with this shit, I'll emigrate'."

At the same time as leaving school, her first play, Ariel 15, appeared on a Berlin stage before being broadcast on German radio. Soon after she made her first film, a youth drama called Torpedo, which won a major prize for newcomers. "It was a good thing to have done because it proved I wasn't a loser as many suspected, and that I had the stamina to see something through."

Then came Axolotl Roadkill, written quickly and in snatches. The title refers to a tiny Mexican amphibian, which never becomes a full adult, a sort of Peter Pan of the animal world and the perfect metaphor for her chief protagonist who proclaims: "I know exactly what I want: not to grow up."

It is not autobiographical, insists Hegemann somewhat unconvincingly, but similar to her own life, telling the story of Mifti, who has been abandoned. Her mother is dead, her father absent. She refuses to go to school, and drifts through the city, searching for fellow drug users, clubbing incessantly, having sex whenever it's on offer and falling in love with a much older woman in whom she clearly sees a mother substitute.

The novel's descriptions are graphic and sometimes rather stomach-churning. Little actually happens in the novel, which is nevertheless eclectic in the sweep of topics it covers, veering between discussions on Foucault to the lancing of Karl Marx's boils. While Mifti's precociousness does not initially endear her to the reader, the way she conveys the tragedy of an abused child with adult incisiveness and quick-witted prose gradually draws one in.

Then came the fall, as spectacular and dramatic as her rise to literary celebrity had been, when a blogger revealed that material in the novel had been lifted from the much less well-known novel Strobo, written under the nom de plume Airen. Other unattributed sources subsequently came to light, and the early gushing praise turned to a deluge of indignation and opprobrium.

Hegemann is apologetic but only to a point. "There were 14 sentences that I took from a blogger and modified," she says. "Suddenly they were rubbishing the entire book. Critics who had feted me were suddenly distancing themselves completely. But I think they were looking for a way to neutralise me and debilitate me as an author, and the plagiarism claims were the approach path for allowing them to do that.

"But I've said it again and it's still my best defence: there's no such thing as originality, just authenticity," she says.

I cite the most infamous of the lifted passages, near the start of the book, where Mifti's brother Edmond tells her: "Berlin is here to mix everything with everything, man … I steal from anywhere that resonates with inspiration or fuels my imagination … because my work and my theft are authentic as long as something speaks directly to my soul. It's not where I take things from – it's where I take them to."

The thoughtful and precise English translation by Katy Derbyshire has been slightly but very noticeably altered from the German, to have Mifti pointedly ask her brother: "So you didn't make it up?" To which he replies: "No, it's from some blogger." The modified remark is intended, thinks Hegemann, to make clearer the quotation's origin.

"But you know the joke of it is, they say I stole that sentence from Airen, when in fact it's originally from Jim Jarmusch, who I think saw it on a gallery sign somewhere, and then the line 'I steal from anywhere' is Jarmusch quoting Jean-Luc Godard." She beams as she tells me this.

Throughout the interview she speaks in long, complex sentences sprinkled with the odd English word ("surrounded", "shit", "nightlife"). She says how stressful the accusations were and that her publisher, Ullstein, offered her little protection. "It is ironic that what I was accused of 'stealing' had already been 'stolen'," she adds, segueing into an explanation about intertextuality and how words and phrases percolate into works unconsciously.

In a way the English version of her novel has been something of a refreshing experience, a way of presenting the book anew. (All quotations are now sourced, filling around six pages of text.) Hegemann feels that she is being taken seriously again. It means much to her that translation rights have been sold to 20 countries.

Looking back, though, what was it like to feel the wrath of the Nobel laureate Grass? Hegemann smirks and says with self-assuredness one might expect from someone twice her age: "I was able to look on that with an element of humour because it was like the oldies rebelling against the young, and it wasn't a battle I had sought but I thought: 'Bring it on!' I was also amused that I was being accused of dastardly deeds by someone who did God knows what when he was 17," she says, mischievously referring to Grass's belated admission that he had been in the Waffen-SS.

To what extent Hegemann experienced the events and incidents in the book herself – there are graphic references to gastric excesses, puke-encrusted clothing, anal sex, drug taking (whizz, ketamine, phencyclidine) – she will not be drawn.

"Everyone wants me to admit it's my life as I've experienced it, but I used this girl in Berlin and its nightlife as a vehicle, as it's a life I know about. I couldn't write about a Brazilian store detective because I know nothing about his life. Anyway, even if I did have these experiences, I would not admit it ... details of my life are not something I'd want to bore someone else with," she insists.

Hegemann has kept a relatively low profile since the book's publication. She was living in a "capitalist-compatible commune" but has recently moved into her own flat near Berlin's central station, with her pet dog Charlotte, a stray she found on the streets of a Belgrade suburb. "That's one of the advantages of writing a bestseller," she says. "It hasn't made me rich, but it's enabled me to get my own place and a washing machine for the first time in my life."

She's writing plays and working on a new novel about which she is sworn to secrecy. "All I can say is that it's not set in Berlin," she says. "And it's driven by my credo not to write anything that doesn't conform with my personal, political or societal convictions."

Looking back on how Axolotl has changed her life, Hegemann becomes pensive. "I'm still very angry about what happened and how I was left on my own to deal with all the crap surrounding it, and how everyone seemed to be saying I'd stolen something because I was too uncool, ugly and stupid to think it for myself," she says.

But one of her lingering achievements has nothing to do with literature but rather the indirect way she has served the cause of a hitherto little-known salamander. "There are five times more axolotls being kept as pets in Germany than before my book," she says. "It's great to achieve such concrete results with your shite book."